The present invention relates to a system in a supercharged engine for controlling the quantity of fuel to be injected into an intake system of the engine.
It is well known to introduce a portion of the suction air, which has been compressed by a supercharger, into the exhaust system of the engine as a secondary air for the substantial purification of exhaust gases emitted from the engine. In the prior art engine system embodying this technique, the quantity of air actually introduced into the engine cylinder is of a value less the quantity of air introduced into the exhaust system, i.e., the quantity of the secondary air, and therefore, the fuel to be injected into the engine cylinder must be reduced in a quantity appropriated by the reduction in quantity of the air being introduced into the engine cylinder.
To this end, the prior art engine system such as disclosed in, for example, Japanese Laid-open Patent Publication No. 55-153817 published Dec. 1, 1980, makes use of primary and secondary air flow sensors: the primary air flow sensor is installed on the intake system downstream of the air cleaner for detecting the total flow of the air towards the engine cylinder, and the secondary air flow sensor is installed on a secondary air supply passage, connecting between the intake system and the exhaust system, for detecting the flow of the secondary air towards the exhaust system. In practice, the primary air flow sensor is constituted by an air flowmeter generally utilized in an engine system of fuel injection type, whereas the secondary air flow sensor is similar in structure to the air flowmeter. This prior art engine system also makes use of a microprocessor operable to perform a subtraction between the total air flow, detected by the primary air flow sensor, and the secondary air flow detected by the secondary air flow sensor for the determination of the actual quantity of air actually introduced into the engine cylinder and then to control the quantity of fuel to be injected in dependence on the actual quantity of the air introduced into the engine cylinder.
It has, however, been found that since the secondary air flowing through the secondary air flow sensor is a portion of the air compressed by the supercharger, and, therefore, evolves a high temperature while the pressure thereof often fluctuates, the secondary air flow sensor is placed under severe conditions and is susceptible to trouble. Once the trouble occurs in the secondary air flow sensor, the flow of the secondary air is no longer metered accurately and, as a result, no accurate control of the quantity of fuel to be injected can be achieved. In addition, when the engine is being operated under a transit condition intermediate between the conditions in which the supply of the secondary air into the exhaust system is required and is not required, respectively, the supply of the secondary air tends to become unstable to such an extent as to result in difficulty in accurate detection of the quantity of the secondary air being introduced into the exhaust system.